Mechanical kinetic vacuum pump with rotor and shaft

ABSTRACT

A mechanical kinetic vacuum pump with a stator ( 1 ), a rotor ( 6, 7 ) made from an aluminum or magnesium alloy and a rotor ( 6, 7 ) and a bearing shaft ( 3 ). The shaft ( 3 ) and the rotor ( 6, 7 ) are connected by a shrink- or screw-fit in a secure permanent connection. The rotor ( 6, 7 ) is made by spray forming. The main alloying component in the aluminum or magnesium alloy is silicon which is present in an amount such that the alloy has an expansion coefficient which essentially corresponds to the expansion coefficient of the shaft material.

BACKGROUND OF THE INVENTION

The present invention relates to mechanical kinetic vacuum pumps. Itfinds particular applications to vacuum pumps in which the rotor andstator are connected by a shrink-fit or a screw-fit.

By definition gaseous ring vacuum pumps, turbo vacuum pumps (axial,radial) and molecular/turbomolecular pumps belong to the class ofmechanical kinetic vacuum pumps. They are capable of mechanicallytransporting within the molecular flow range (pressures below 10⁻³ mbar)the gas particles which are to be pumped. Moreover, molecular pumps arealso capable of pumping gases within the Knudsen flow range (10⁻³ to 1mbar). Presently employed mechanical kinetic vacuum pumps frequentlyoffer a turbomolecular pumping stage and a downstream molecular pumpingstage (compound or hybrid pump), since such pumps are capable ofcompressing gases up in to the viscous flow range.

Pumps of the kind affected here, in particular turbomolecular vacuumpumps are operated at high rotational speeds up to 100,000 rpm. Thisrequires a firm and tight joint between rotor and shaft which meets therequirements regarding rotor dynamics when passing through criticalspeeds. The joint commonly is provided by a shrink- or screw-fit. Theshrink-fit joint is provided by joining the warm rotor and the cooledshaft, in that the shaft is introduced into a bore in the rotor.Generally steel is employed as the material for the shaft, since steelhas a relatively high modulus of elasticity. For reasons of rotordynamics a lighter material, preferably aluminium, is employed as therotor material. Here aluminium alloys produced by melt metallurgy, e.g.,casting, are well proven. However, in the instance of the material pairof steel/aluminium it is difficult to implement a joint between rotorand shaft which is free of backlash and settling since the coefficientsof expansion of steel (about 11×10⁻⁶/K) and aluminium (about 22×10⁻⁶/K)differ.

From U.S. application Ser. No. 09/937,876, it is known to achievefreedom from backlash and settling at the joint between rotor and statorby providing reinforcement rings preventing an expansion of thealuminium rotor which would give rise to backlash. These measures areinvolved engineering-wise.

SUMMARY OF THE INVENTION

It is the task of the present invention to create a mechanical kineticvacuum pump in which a firm joint between shaft and rotor is attained bymore simple means.

This task is solved by alloying the rotor to have an expansioncoefficient which essentially corresponds to the expansion coefficientof the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

The FIGURE is a side-sectional view of a vacuum pump in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aluminium alloys produced through powder metallurgy (for example,through spray forming) are basically known in other applications. Theseare manufactured such that the melt consisting of the alloy'sconstituents is sprayed by nozzles on to a cold surface. Compared to themelt metallurgical manufacture of aluminium materials, e.g., casting,the melt solidifies very rapidly through which the alloy attains a newstructure with changed properties. Aluminium alloys manufactured byspray forming, with the principal constituent being silicon, may be soadjusted that their coefficient of expansion corresponds to that ofsteel.

In that there is no or only a slight difference between the coefficientsof expansion of shaft and rotor, loosening of the shrink-fit jointbetween shaft and rotor under the influence of temperatures in theoperational state is prevented. Equally a joint offering a reducedshrink-fit tension may be manufactured which is easier to join and whichincurs less strain on the material. Also, the bore and shaft can bemanufactured with greater tolerances which—just like the more simplejoining process—causes manufacture to be less involved and thus lesscostly.

The present invention shall be explained in the following with referenceto the pump of the kind affected here depicted in the FIGURE. The pumpdepicted has an outer housing 1 with a central bearing sleeve 2penetrating inside the housing. The bearing sleeve 2 supports the shaft3 by means of a spindle bearing arrangement 4. Drive motor 5 and therotor system 6, 7 are linked by shaft 3.

The single-piece rotor has two rotor sections 6 and 7 differing indesign. Rotor section 6 is cylindrical in shape with a smooth outer andinner surface 8, 9. In the area of the surface 8, the housing 1 isequipped on its inside with a thread 10, thus at the same time formingthe stator of a screw pumping stage. The surface 8 and the thread 10 arethe active pumping surfaces of a screw pumping stage which is basicallyknown and which pumps molecules entering into the pumping slot 11towards the direction of the outlet 12.

In the area of the inner surface 9 of the rotor section 6 the outside ofthe bearing sleeves 2 is equipped with a thread 13 and thus forms thestator of a further screw pumping stage. The thread 13 and the innersurface 9 are the active pumping surfaces of the further screw pumpingstage with the pumping slot 14. The gases being pumped from the bottomto the top through pumping slot 14 flow to the outlet 12 through bores15 in the bearing sleeve 2.

Located upstream of the screw pumping stage 8, 10 is a further pumpstage. This has a rotor section 7 which consists of a cone-shaped hubcomponent 23 and the ridges or vanes 24. These ridges 24 form, with thestator wall 25 surrounding them in housing 1, a pump stage 7, 25. Gasmolecules entering between the individual ridges 24 or into the slot 26are pumped by the pump stage 24, 25 in the direction of pumping slot 11of the molecular pumping stage 6, 10.

The shaft 3 carries the rotor section 7 which in turn carries the rotorsection 6. The cylindrically shaped rotor section 6 may, but not must,consist of the same material as for rotor section 7. The employment ofcylinder sections containing carbon fibres, for example, in the rotorsof molecular pumps is also possible. The joint between shaft 3 and rotorsection 7 is produced by a shrink-fit.

If the shaft 3 is fabricated of steel and the rotor system 6, 7—or atleast rotor section 7—is fabricated of the alloy in agreement with thepresent invention, then the coefficients of expansion of shaft 3 androtor 6, 7 are equal or almost equal. Even in the instance of hightemperature loads on the rotor, which occur in particular when employingthe pumps affected here in the semiconductor industry, a secure joint ofrotor and shaft is ensured.

Materials of the types according to the present invention are beingoffered on the market under the names of DISPAL (DISPAL A/S 230, DISPALS241, A and S 250, for example). Besides aluminium they contain 16 to 22percent in weight silicon as the main constituent as well as other alloyconstituents like iron, nickel, copper, magnesium, and/or zircon atshares of between 0.3 and 8 percent in weight.

In a material of comparable properties, a different light materialnamely magnesium may be present instead of the aluminium base material.Thus the advantage detailed for alloys based on powder metallurgy may bealso utilised for alloys based on magnesium. The coefficient ofexpansion may be adjusted through suitable additional constituents likeSi, for example.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A mechanical kinetic vacuum pump including: a stator, a rotor sprayformed from an aluminum alloy, a bearing shaft made of a shaft material,the bearing shaft connected directly to the rotor with a shrink fitconnection, the aluminum having a main alloying component of silicon inan amount such that the rotor has an expansion coefficient whichessentially corresponds to an expansion coefficient of the shaftmaterial.
 2. The pump according to claim 1, wherein the silicon ispresent in the alloy in amounts of 16 to 22 percent by weight.
 3. Thepump according to claim 1, wherein the rotor material contains furtheralloy constituents, including at least one of iron, nickel, copper andzirconium.
 4. A mechanical kinetic vacuum pump comprising: a rotor madeof one of a magnesium alloy and an aluminum alloy and manufactured bypowder metallurgy; a rotor shaft; and a shrink-fit connection connectingthe rotor to the shaft.
 5. The pump according to claim 4 wherein therotor alloy includes silicon present in an amount such that the rotoralloy has an expansion coefficient which is the same as an expansioncoefficient of steel.
 6. The pump according to claim 5 further whereinthe rotor shaft is steel.
 7. The pump according to claim 5 wherein therotor alloy further includes at least one of iron, nickel, copper, andzirconium.
 8. A method of manufacturing a mechanical kinetic vacuumpump, the method comprising: spray forming a light metal alloy into arotor, the rotor including a central bore; at least one of heating therotor and cooling a rotor shaft to create a thermal differencetherebetween; inserting the rotor shaft into the rotor bore such that asthe rotor and the shaft are brought into thermal equilibrium, the rotorcentral bore shrinks relative to the shaft forming a shrink-fitinterconnection; fitting the rotor with the interconnected rotor shaftinto a stator.
 9. The method according to claim 8 wherein the lightmetal alloy is one of an aluminum alloy and a magnesium alloy.
 10. Themethod according to claim 9 wherein the alloy includes silicon in anamount such that the rotor has a common expansion coefficient with theshaft.
 11. The method according to claim 10 wherein the shaft is steel.12. The method according to claim 11 wherein the silicon is present inan amount of 16 to 20% by weight.
 13. The method according to claim 10wherein the alloy further includes at least one iron, nickel, copper,and zirconium.
 14. A mechanical kinetic vacuum pump manufactured by themethod of claim 8.